1. Field of the Invention
This invention relates to stabilizing compositions, elastomers containing such compositions and methods for stabilizing the elastomers. More particularly, this invention relates to compositions for stabilizing elastomers against degradation comprising synergistic mixtures of triaryl, trialkyl phosphites and mixtures thereof, and a nucleophile, heat-treated technical lignins.
2. The Prior Art
Most organic polymeric materials are prone to atmospheric degradation during manufacture, processing, storage and in service. Such polymers include natural and synthetic elastomers as well as plastics, and particularly those polymers which contain processing or catalytic residues therein and which are subject to rapid degradation in physical properties upon use or exposure to heat, light, or atmospheric oxidants. The rate of degradation depends primarily on the composition of the polymer and the conditions of exposure to oxygen, ozone, heat, and light. These adverse effects may be greatly retarded in most materials by the use of stabilizers and antioxidants. Stabilizers and antioxidants then are substances that retard the degradative effect of oxidation and thereby extend the polymer's useful temperature range and service life when added in low concentrations. For the purpose of this specification the terms "stabilizer" and "antioxidant" will, for the sake of simplicity, by referred to interchangeably. The concentration employed depends on the structure, processing conditions, impurities and end use of the polymer. It is common practice to include in polymers, such as elastomers, small amounts of from about 0.01 to 5.0 percent by weight of an antioxidant.
J. R. Sheldon "Rubber Chemistry and Technology", Vol. 45, No. 2, Apr. 15, 1972, pages (356-380) reviews the mechanisms of oxidation and antioxidation. Antioxidants act by inhibiting the formation of free radicals in the initiation step or by interrupting the propagation cycle. The formation of free radicals may be prevented by light absorbers, metal deactivators or peroxide decomposers. Materials that interrupt the propagation cycle are called free radical scavengers or inhibitors. The mechanisms by which the stabilizers and combination of stabilizer used in this invention perform are directly related to the free radical chain reaction described in the Sheldon reference. Murray and Watson "India Rubber World", Vol. 118, August 1948, pages (667-669) teach that unoxidized soda pulp lignin is a stabilizer for styrene-butadiene rubber (SBR). However, such lignin is not presently used as a stabilizer.
The preparation of triaryl and trialkyl phosphites and their use alone as rubber stabilizers is well known having been described in various forms in a number of patents including U.S. Pat. No. 2,419,354 to Howland et al. and U.S. Pat. No. 2,733,226 to Hunter. More exotic substituted triaryl phosphite esters to stabilize rubber compounds are disclosed in U.S. Pat. No. 3,112,286 to Morris et al.
In addition to the use of lignin alone as a stabilizer and the use of phosphites alone as stabilizer described above, blends of phosphites in combination with certain phenolic-type materials are described in U.S. Pat. No. 3,115,465 to Orloff et al., who teaches inhibiting oxidative deterioration of hydrocarbon fuels, oil, etc., with a mixture of soluble phosphite esters and methylene bis-phenol. Another patent, U.S. Pat. No. 3,535,277 to Miller et al., disclosed blends of an alkyl substituted aryl phosphites and certain phenols that provide stabilization properties for polymers.
G. Scott (Atmospheric oxidation and Antioxidants, Elsevier, 1965, p. 203) discusses synergism. Scott states: "although synergism exists if the overall effect of (a mixture of) two antioxidants is better than that which would be expected on the basis of a summation of the effects of each component alone, in practice the term is frequently used . . . to describe an effect which is better than the more powerful antioxidant at the same overall conventration. For convenience, then the latter effect will be referred to as practical synergism . . ."
Combinations that exhibit practical synergism are the subject of this invention. Therefore, it is the general object of this invention to provide synergestic compositions which are highly effective as stabilizers of elastomers against atmospheric degradation. Another object of this invention is to provide elastomers that have been stabilized against degradation. Still another object of this invention is to provide a method of stabilizing elastomers. An even further object of this invention is to accomplish the above objects with an additive composition comprising triaryl phosphites or trialkyl phosphites, and mixtures thereof and a nucleophile, heat-treated technical lignin that exhibits a synergistic effect against oxidative degradation.
Other objects, features and advantages of this invention will become evident from the following detailed description of the invention.